Fixing Arb.localDate when minDate == maxDate (kotest#3860)

Now simply returns a constant Arb for the given date.

Fixes kotest#3829

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kotest-property/src/jvmMain/kotlin/io/kotest/property/arbitrary/dates.kt

@@ -55,16 +55,19 @@ fun Arb.Companion.localDate() = Arb.Companion.localDate(LocalDate.of(1970, 1, 1)
 fun Arb.Companion.localDate(
    minDate: LocalDate = LocalDate.of(1970, 1, 1),
    maxDate: LocalDate = LocalDate.of(2030, 12, 31)
-): Arb<LocalDate> {
+): Arb<LocalDate> = when {
+   minDate > maxDate -> throw IllegalArgumentException("minDate must be before maxDate")
+   minDate == maxDate -> Arb.constant(minDate)
+   else -> {
+      val leapYears = (minDate.year..maxDate.year).filter { isLeap(it.toLong()) }
 
-   val leapYears = (minDate.year..maxDate.year).filter { isLeap(it.toLong()) }
+      val february28s = leapYears.map { LocalDate.of(it, 2, 28) }
+      val february29s = february28s.map { it.plusDays(1) }
 
-   val february28s = leapYears.map { LocalDate.of(it, 2, 28) }
-   val february29s = february28s.map { it.plusDays(1) }
-
-   return arbitrary(february28s + february29s + minDate + maxDate) {
-      minDate.plusDays(it.random.nextLong(ChronoUnit.DAYS.between(minDate, maxDate)))
-   }.filter { it in minDate..maxDate }
+      arbitrary(february28s + february29s + minDate + maxDate) {
+         minDate.plusDays(it.random.nextLong(ChronoUnit.DAYS.between(minDate, maxDate)))
+      }.filter { it in minDate..maxDate }
+   }
 }
 
 /**

kotest-property/src/jvmTest/kotlin/com/sksamuel/kotest/property/arbitrary/DateTest.kt

@@ -1,6 +1,7 @@
 package com.sksamuel.kotest.property.arbitrary
 
 import io.kotest.assertions.throwables.shouldNotThrowAny
+import io.kotest.assertions.throwables.shouldThrow
 import io.kotest.core.spec.style.WordSpec
 import io.kotest.inspectors.forAll
 import io.kotest.matchers.collections.shouldContain
@@ -82,6 +83,20 @@ class DateTest : WordSpec({
       }
    }
 
+   "Arb.localDate(minDate, maxDate)" should {
+      "Work when min date == max date" {
+         val date = of(2021, 1, 1)
+         Arb.localDate(date, date).take(10).toList() shouldBe List(10) { date }
+      }
+
+      "Throw when min date > max date" {
+         shouldThrow<IllegalArgumentException> {
+            val minDate = of(2021, 1, 1)
+            Arb.localDate(minDate, minDate.minusDays(1))
+         }.message shouldBe "minDate must be before maxDate"
+      }
+   }
+
    "Arb.localTime()" should {
       "generate N valid LocalTimes(no exceptions)" {
          Arb.localTime().generate(RandomSource.default()).take(10_000).toList()